Gunilla Efraimsson

A frequency domain linearized Navier–Stokes equations approach to acoustic propagation in flow ducts with sharp edges

Acoustic wave propagation in flow ducts is commonly modeled with time-domain non-linear Navier-Stokes equation methodologies. To reduce computational effort, investigations of a linearized approach in frequency domain are carried out. Calculations of sound wave propagation in a straight duct are presented with an orifice plate and a mean flow present. Results of transmission and reflections at the orifice are presented on a two-port scattering matrix form and are compared to measurements with good agreement. The wave propagation is modeled with a frequency domain linearized Navier-Stokes equation methodology. This methodology is found to be efficient for cases where the acoustic field does not alter the mean flow field, i.e., when whistling does not occur.

Wake characteristics of high-speed trains with different lengths

Three different train configurations with different numbers of cars are analysed in order to investigate the effect of the train length on wake structures. The train geometry considered is the aerodynamic train model and the different versions have two, three and four cars. Due to the different lengths of the trains, the boundary-layer thickness will be different at the tail of each configuration. The flow is simulated using detached eddy simulation, and coherent flow structures are extracted via proper orthogonal decomposition and dynamic mode decomposition. As a result of reconstruction of the flow field using coupling of the mean flow and the first fluctuating proper orthogonal decomposition mode, it is found that the dominant flow structure in the wake is the same for all three cases. However, this structure has different frequencies and wavelengths depending on the boundary-layer thickness in front of the separation. It is shown that the frequency decreases as the boundary-layer thickness increases for these train configurations.

A Remark on Numerical Errors Downstream of Slightly Viscous Shocks

Lower-order errors downstream of a shock layer have been detected in computations with nonconstant solutions when using higher-order shock capturing schemes in one and two dimensions [B. Engquist and B. Sjogreen, {SIAM J. Numer. Anal., 35 (1998), pp. 2464--2485]. By analyzing the steady-state solution of slightly viscous hyperbolic systems of conservation laws we find that the solution can have an

Elimination of First Order Errors in Shock Calculations

{\cal O}(h)

On Acoustic Multi-Port Characterisation Including Higher Order Modes

-dependence downstream of a shock layer, although the viscous term in that region is of

Detached-Eddy Simulations Applied to Unsteady Crosswind Aerodynamics of Ground Vehicles

{\cal O}(h^2)

Aerodynamic prediction tools for high-speed trains

. Numerical examples illustrate the analysis.

The influence of edge geometry on end-correction coefficients in micro perforated plates

First order errors downstream of shocks have been detected in computations with higher order shock capturing schemes in one and two dimensions. Based on a matched asymptotic expansion analysis we show how to modify the artificial viscosity and raise the order of accuracy.

Numerical Investigation of a Realistic Nose Landing Gear

Methods to design test-procedures for acoustic multi-ports in ducts with a focus on pressure sampling positions for accurate modal decomposition are demonstrated. Acoustic fields up- and downstream ...

Analysis of Flow Structures in the Wake of a High-Speed Train

Crosswind stability is an important safety issue for manufacturers of cars, buses and rail vehicles. Since side wind conditions are unsteady phenomena they require time-dependent techniques to simulate the flow. In this study, a hybrid RANS-LES methods, Detached-Eddy Simulation, is applied to evaluate headwind and unsteady crosswind situations for a simple model of car. A grid refinement study is carried out to evaluate the accuracy of the calculations. Convergence in the force coefficients while refining the grid suggests that a certain level of grid convergence is reached. A similar conclusion is drawn from the unsteady simulations.